In recent years automobile manufacturers have offered as optional equipment rear windows which can be defrosted and/or defogged by use of an electrically conductive grid permanently attached to the window. In order to defrost quickly, the circuit must be capable of supplying large amounts of power from a low voltage power source, for example 12 volts. Furthermore, the lines of the conductive grid must be sufficiently narrow in order to maintain visibility through the rear window.
The resistance requirements of conductive patterns is on the order of 2 to 15 milliohms per square, which requirements are easily met by noble metal conductors, particularly silver, which is currently the most widely used conductor material in this application.
Heretofore, the materials used for the preparation of window defogging grids have mostly been thick film silver conductors which are prepared from paste comprising finely divided silver powder particles and glass frit dispersed in an organic medium. In a typical application, a paste containing by weight 70% silver powder, 5% glass frit and 25% organic medium is screen printed through a 180 Standard Mesh Screen onto a flat, unformed glass rear window. The printed composition is dried for at least 2 minutes at about 150 C and the entire element is then fired in air from 2 to 5 minutes at 650 C. After firing the softened glass is shaped by pressing into a mold and then tempered by rapidly cooling. During the firing cycle the organic medium is removed by evaporation and pyrolysis. The glass and silver are sintered to form a continuous conductive path with the glass acting as binder for the silver particles.
In the manufacture of automotive defoggers, an important criterion has been the appearance of the interface between the glass and conductor pattern--particularly color. By far the most predominant type of glass used in the manufacture of automotive windows is soda-lime glass made by the float glass process in which the molten soda-lime glass is cast upon a long pool of molten tin or tin alloy in a controlled atmosphere. This process produces a glass of nearly perfect flatness and excellent thickness uniformity without the necessity of grinding and polishing. During the float glass process a small amount of tin diffuses into the glass from the bath. This leaves no visible image on the surface of the glass. However, the tin diffusion results in different chemical interactions of the glass with staining metals such as silver and copper and the glass surface is harder.
When silver is used as a conductive material, a naturally dark brown color is produced at the glass-conductor interface even without the addition of colorants when the conductor is printed on the "tin side" of the glass, i.e. on the side of the glass which was next to the tin in the float process.
In the manufacturing process, it would be preferred to print the conductor on the air side of the glass in order to avoid certain handling difficulties. For example, less yield loss would be incurred from mechanical damage to the glass surface and enamel color could be better controlled. However, when the conductor is printed on the "air side" of the glass, i.e. on the side exposed to the atmosphere, little color development takes place. For this reason, the automotive glass industry usually prints silver paste onto the tin side of float glass after printing an enamel border on the glass. Consequently, formulations of thick film pastes for this use have contained various coloring agents so that they can be printed on the air side of the glass with suitable color development.
Frequently used colorants for printing on the air side have been silver salts such as Ag.sub.2 SO.sub.4 and Ag.sub.2 S, which upon firing take part in ion exchange reactions with components of the glass substrate to form particles of silver at the interface between the substrate and the conductor composition printed thereon. Conductor patterns made by this method do, however, have several disadvantages.
(1) Colorants of the above-described kind have to be used in substantial concentrations in order to get adequate color development; PA1 (2) When substantial concentrations of such colorants are used, the surface is considerably less solderable because it is inadequately wetted by the solder; PA1 (3) Frequently, a transparent yellow color is formed in the areas adjacent to the lines because of the reduction of silver ions; PA1 (4) These particles then form colored agglomerates of Ag by ion exchange with the glass, thereby producing a mobile salt species. After prolonged humid storage, the mobile salt species, such as Na.sub.2 SO.sub.4, forms a salt scale on the silver surface; and PA1 (5) The colors developed by the above-described method tend to change upon firing. PA1 (1) screen printing a pattern of the above-described thick film paste onto the air side surface of the glass; and PA1 (2) firing the glass and thick film pattern at 580-680 C. to effect volatilization of the organic medium from the thick film paste and liquid phase sintering of the glass frit.
The invention is therefore directed at reducing the impact of the above referred problems on air-side patterning in the manufacture of automotive window glass.